The main purpose of the proposed study is to investigate in detail the development of visual connections in the rat brain and how the patterns of development are disturbed by extrinsic factors (e.g., trypan blue, lesions) introduced at different stages. The results will be obtained using light electron microscopy and both physiological and behavioral methods. The work done so far suggests that the rat's visual system serves as a good model for early development in primates. Additional work will investigate the effects of squint on callosal connections in cat and monkey. From these studies it would be hoped to derive some general conclusions regarding the nature of interactions of different factors controlling the development of the highly ordered, mature, visual system.